Val Plumwood, who has died aged 68 from a stroke, was an eminent Australian environmental philosopher who lived life on her own terms, often in opposition to prevailing mores. Her book Feminism and the Mastery of Nature (1992) has become a classic.

It was in the 1970s that she became involved in a radical critique of the traditional western concept of nature, in which only human beings mattered and nature was not morally significant. This critique emerged at opposite edges of the western world - in Norway and Australia. The Norwegian critique emanated from philosopher Arne Naess, who became the founder of the deep ecology movement. The Australian ideas had their origins in the work of a small group of philosophers at the Australian National University (ANU) in Canberra, of which Plumwood was a key member.

Both Naess and the ANU group recognised that the world's environmental problems were the upshot not merely of faulty policies and technologies, but of underlying attitudes to the natural world built into western thought, reflecting the belief that only human beings mattered, morally speaking, in their own right. If we chose to conserve nature rather than exploit it, this was only for the sake of the utility it would have for future human generations. Humanity was under no moral obligation to preserve nature for nature's sake.

Together with her second husband, the philosopher Richard Routley, Plumwood asked a question that became the title of one of their most famous papers: "Is there a need for a new, an environmental, ethic - an ethic of nature?" She dedicated the rest of her life to exploring this question. She saw that anthropocentrism, as a value system, rests on the assumption that there is a deep division between humanity and nature: human beings, though embodied and, in that sense, material, are somehow different in kind from the rest of material nature. The "something" that sets us apart is, of course, mind. The division between mind and matter becomes refined, in the western tradition, into an opposition between reason and nature.

Plumwood provided an analysis of how this division had historically been constructed, and how it informs many of the categories of western thought. She showed how this dual system rendered inferior all the terms that came to be associated with nature rather than reason - women, the working-class, the colonised, the indigenous, as well as the other-than-human world. She thereby demonstrated that such an ideology legitimised the domination of many subjugated social groups. The implication is that environmentalism and struggles for social justice cannot be separated from one another.

Plumwood's understanding of nature's agency was drawn from an intimate engagement with the community of life about her. She was born Val Morrell to poor parents who ran a poultry farm surrounded by forests near Sydney. In the 1970s she and Routley built a stone house in the heart of a temperate rainforest in southern Australia. After their divorce in 1981, she lived there alone, though she saw herself as a member of a convivial biotic community of animal and plant beings who provided daily drama, delight, excitement and opportunities for discovery. The deep knowledge of botany, animal behaviour and local natural history she gained from this way of life was legendary.

It was in the wake of her divorce that Val adopted the name of Plumwood, after Plumwood mountain, where her home was. Fittingly, perhaps, the mountain itself was named after the Plumwood tree - a tall and beautiful rainforest species that grew there. Similarly, Routley took the name of Sylvan, under which his later work was published.

Ever critical of academic orthodoxy, Plumwood maintained her independence as a scholar. She had studied philosophy at the University of Sydney in the 1960s, and thereafter taken intermittent teaching posts, at Macquarie University, Sydney, Murdoch University, Perth, the University of Tasmania, North Carolina State University and the University of Montana. In 1991, she was awarded a PhD by the ANU, where, at the time of her death, she was a visiting fellow in the Fenner school of environment and society. Not content with theorising the terms of nature's oppression, Plumwood also fought environmental battles. In the 1970s and 1980s, she was a pioneer in the campaign to save the eastern Australian rainforests - she and Routley wrote the seminal book The Fight for the Forests (1973) - and she continued to struggle in the fight to protect Australia's old-growth forests.

Famous for her fearlessness, she was tested to the limit in 1985 when she was attacked by a crocodile while kayaking alone in the Kakadu national park, in Australia's Northern Territory. After being rolled over three times, then released from the crocodile's jaws, she crawled for hours through tropical swamps, with appalling injuries, before being rescued. The experience made her well placed to write about death and its part in nature, and much of her later work revolved around this theme.

Of Plumwood's many legacies, perhaps the main one for me, as a long-time colleague and friend, was her integrity. Caring little for convention, she unapologetically and energetically lived the life she theorised and never failed to speak out on behalf of non-human others who came into her path. Whether it was for wildlife killed on local roads, rare orchids destroyed by mowing in the cemetery near her home, or crocodiles threatened with hunting in northern Australia, she always made her voice heard. In this way, she showed how philosophy could not only diagnose the world's ills but become something more than a charade of words - a way of life.

A son and a daughter from her first marriage predeceased her.

· Val Plumwood, philosopher and activist, born August 11 1939; died February 29 2008